Beat frequency indicator



2 She`ets-Shet 1 R. D. scHELDORF BEAT FREQUENCY INDICATOR Filed March 31. 1937 Snventor Ranald D. 'cheldorf Bg y ffomeg April 3o, 1940.

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April 3.0, 1940. R. D. scHELDoRF l BEAT FREQUENCY INDICATOR Filed March 3l, 1937 2 Sheets-Sheet 2 Ranald D. 'cheldorf y (ttorneg Patented-Apr. 30, 1940 Ranald signor to Radio poration of Delaware Scheldorf, Haddon Heixhts, N. J., 4ail- Corporatlon of America, a cor- Application March 31, 1937, Serial No. 134,092

4 Claims.

My invention relates to frequency indicators. Specifically, my invention provides means for indicating the frequency relation of two alternating currents, one of which, for example, may 5 have a fixed known frequency and the other may have a variable frequency.

By way of illustration, a particular application will be described for which my invention is .par-

ticularly suitable, but it is evident that there are other devices which may well incorporate the frequency indicator according to my invention.

Audio frequency currents which are adjustable throughout a range of frequencies are often produced by combining currents from two oscillators.

One of the oscillators is fixed-inl frequency, while the other oscillator is adjusted throughout a range of frequencies to obtain the desired beat frequency oscillations. Such a device is commonly known as a beat frequency oscillator. It is customary to use a frequency adjusting element e which is'calibrated in cycles per second or .in

other conventional'units. In order to have the calibration of this element coincide with the actual beat frequency, .a Vernier provided. By means of the Vernier the operator' adjustment is of the device adjusts the audio output frequency until an indication is obtained that second, I propose the output frequency is equal to a known reference frequency.

Several different arrangements posed to determine the position at nier adjustment should. be placed to have the calibrated scale suitably match the audio output is to use a tuning frequencies. One such means 'fork to compare the tuning fork have been prowhich the verfrequency with the audio frequency output. Another means is to provide The galvanometer is actuated by a frequency current and a portion o a galvanometer as. the indicating source.

standard audio f the generated audio frequency output to indicate beat frequencies.

My invention provides a very simple'means of indicating the actual frequency Inasmuch as the of 'the output.

commercial power source for the' oscillators, demodulator and audio-frequency amplifier is usually a .current of cycles per l to provide means for comparing the 60cycle per second supply current with the unknown audio frequency generated by the The means which Il beat frequency oscillator.

propose to employ has a number low cost, permanence,

such as simplicity,

of advantages absence of fmoving parts, continuous indication, high sensitivity, low operating power,

and ability A to correctly indicate harmonic relationships which quency oscillator I equal to the` difference frequency and the sum` upon the input tend to give false indications in some of the indicators which have been previously employed.

v One Aof the objects of my invention is to provide means for indicating the frequency of variable frequency current as compared with a substan- 5 frequency indicating means which is inertialess, inexpensive and simple to operate.

Referring to the accompanying drawings',

Figure 1 is a schematic illustration of one embodiment of my invention as applied to a beat frequency oscillator,

Figures 2a, 2b, 2c,

2d and 2e are illustrations of patterns of the in cator of Fig. 1, and

Figure 3 is a schematic circuit diagram showing one method of applying my invention.

In Fig. 1 the output circuit of a fixed freis connected to a resonant transformer system 3. The transformers 3 terminate in a pair of attenuators 5, 1. The adjustable elements of the attenuators 5, 1 are connected to the input circuit of a push-pull demodulator 9. An adjustable frequency oscillator II, which includes a frequency adjusting means I3 and a Vernier adjustment I5, is con- 30 nected to the input circuit of the demodulator 9 so that the output from the variable frequency oscillator II is impressed upon the demodulator circuits in parallel. The output of the demodulator 9 includes currents whosev frequency is frequency of the flxed and variable frequency oscillators and the freque of the fixed frequency oscillator.` Of these several components theaudio frequency current, which is equal yto the diilerence frequency, is passed and the radio frequency currents attenuated by a suitable filter I1. The audio frequency currents are impressed circuit of a push-pull amplifier I9. The output circuit from the audio frequency amplifier is represented by a pair of terminals 2 I.

The power supply for the foregoing apparatus is derived vfrom a sourcev 23 of alternating current. The alternating current is rectified and filtered by a suitable means 25 and. impressed upon the anode circuits of the thermionic tube included in the oscillators, demodulator and audio frequency amplifier.- Aportion of the (iO-cycles per second current pressedb upon the beat which preferably includes an amplifier. A portion ofv the audio frequency output current is likewise impressed on the beat frequency indicator 29.

One .suitable indicator is manufactured under the trade-mark name Magic Eye. This device includes an amplifier tube and a wlllemite screen 30 upon which the effect of applied potential indications may be observed in terms of a fluorescent area. If the audio frequency output current is of exactly the same frequency as the 60a cycles per second supply current, and of the opposite phase, and if these two currents are at substantiallythe same potential, the indication will be a steady angular opening, or sector, 32 which is less luminescent than the fixed area 34 as illustrated in Fig. 2b. 'I'he degree or angle of the opening is determined by the relative phases of the currents. In any event, if the phase is not shifting, the illuminated sector `will belsteady, although it may be included within a very small angle as determined by the tube construction, the maximum angle, or any intermediate angle. If, however, the two frequencies are not exactly the same, but substantially so, the opening of Fig. 2b

will be characterized by a varying increase and' pairs of traces 33, 35 will be increasingly more luminescent. The one trace 32 indicates the 'absence of varying currents, while the pairs of superimposed traces 33, 35 indicate the presen'ce lof second harmonic or 1Z0-cycles per second currents.

If the 60cyc1es per second supply current is l compared with a 30cycles per second audio frequency current, the luminescent sector on the fluorescent screen of the indicator will appear as indicated in Fig. 2a. I'his ligure corresponds with Fig. 2c, except that the lighter trace or sector is now numbered 31 and as. before corresponds to the absence of 'alternating currents,

while the two darker or superimposed pairs of traces 39, 4l indicate the second harmonic relationship, corresponding, respectively, to areas If the 60cyc1es per second alternating current from the source is compared to an audio frequency output of 180 cycles, seven angular traces appear as shown in Fig. 2d. Of these, three on either side of the central trace are of the superimposed type. In a similar manner, if the supply current is compared with 24U-cycles per second audio frequency output currents, nine traces appear as shown in Fig. 2e. If inthe comparison any of the several currents, represented by the illustrations of Figs. 2a, 2C, 2d and 2e', One 0f the currents happens to vary slightly in frequency from its exact harmonic or sub-harmonicrclation to the other, the area covered `by the several superimposed traces will vary cyclically at a corresponding frequency. While I have illustrated 'a few of the patterns which may be obtained and readily' recognized, other more complicated patterns are obtained whenever the two currents bear a definite predetermined fractional or multiple relation to one another. In each instance the relative area covered by the several traces will vary with the phase relation of the two currents. When this variation takes place the traces appear to rotate. The pattern for each particular ratio is different, and the less complex patterns are easily recognizable. Thus, the indication enables the user to determine the frequency of the audio output currents as compared with the 60cyc1es per second supply current.

In general, the indicatibn which is most useful is represented by Fig. 2b. To obtain this indication for the purpose of Calibrating the beat frequency oscillator, the frequency adjustable element I3 is placed adjacent a scale indication of 60 cycles per second. The Vernier 15 is slowly adjusted until the pattern of Fig. 2b is indicated. As this condition is approached, the indicator will show an angular opening in the fixed luminous area which at rst varies rapidly'and gradually slowly changes until finally the area will remain stationary over'a long period of time. This will indicate that the audio output current is set at 60 cycles per second. 'I'hen the 60cyc1es per second audio output current is coacting with the 60cyc1es per second supply current in the indicator to form a so-called zero beat indication,r

and the scale calibration of I3 is therefore correct. If the user has by chance adjusted the Vernier to obtain beats between the 60-cycles per second supply current and currents'of 30, 120, 180 or 240 cycles per second, the more complex patterns shown in Figs. 2a, 2c, 2d and 2e, respectively, will appear. Higher ratio patterns are morediiicult to distinguish, and are therefore not commonly used.

Inasmuch as the initial 60cyc1es per second audio output may be obtained when the variable frequency oscillator of the beat frequency oscil- -lator differs by 60 cycles per second more than or less than the frequency of the xed oscillator,

.it is possible to adjust the Vernier so that the same indication is obtained under either condition. I prefer to avoid this possibility by making the adjustment of the vernier l5 in the direction corresponding to reducing the highest frequency to the lowest frequency within the range of the Vernier, when the main frequency 4adjustment f has been set upon the calibration of 60 cycles per second. In any event, if this error does occur, it may be readily checked by observing the beat frequency indications at 120, 180, etc., cycles per second. 'I'he ability to checkthe higher frequency points in this manner is one of the outstanding advantages of my invention.

Although numerous circuit arrangements may be employedA with the beat frequency indicator of my invention, I prefer to use a circuit similar to the circuit illustrated in Fig. 3. In this circuit, the variable frequency oscillator 43 is arranged with a resonant plate circuit which is suitably coupled to the grid circuit to obtain high frequency current' oscillations which are impressed through a coupling capacitor 45 on the grid electrodes of a pair of `push-pull demodulatorsv 41. The fixed frequency oscillator 43 is also a tuned plate oscillator in which the plate circuit is suitably coupled to the grid circuit to generate oscillatory currents. The variable tuning element is represented as a variable capacitor 5l. Tl'ie output currents of the oscillator 49 are impressed on the primary 53 of the'resonant transformer 55.

'is coupled through a The secondary circuit 51 of the transformer 55 link circuit to a pair of sec- The secondary coils 59 are ondary coils 59.

6|. The adjustable conshunted by attenuators tacts of attenuators to the grid electrodes of the demodulators 41. It. will be observed that the transformers have windings which may be electrostatically shielded by shields 63. It will also be noted that the oscillators are electronically coupled to their output circuits. This type ofcoupling reduces the interaction of the oscillators on each other as the frequency of the two oscillators becomes nearly equal and prevents 'the oscillators from locking in. The adjustable Vernier 65 is used to align .the variable frequency osc'llator in relation tothe fixed frequency oscllator so that the calibration on the scale agrees with the audio frequency output.

The output circuits of the push-pull demodulator 41 are connected to a filter network 61 which may be of any type suitable for attenuating the undesired radio frequency currents and passing the audio frequency interaction of currents of the different frequency of the two oscillators. The audio frequency currents which appear in the output circuit of the filter 61 are impressed upon a push-pull audio frequency amplifier 69. While the amplifier is illustrated as employing four tubes, it should be understood that any suitable audio amplifier may be used. The output of the amplifier 69 is impressed through a transformer 1I on a work or load circuit which is not shown but which is -represented by a series of taps on of the transformer 1|.

The power supply is obtained from a commercial-source of alternating current which is not shown. The plug 15 is connected to the alternating current source and to transformer primary 11. The secondaries of the power trans former are connected to a full-wave rectifier 19 and regulator tubes 8l, 83. The purpose of the regulator tubes is across the potentiometer network 85. Although there are numerous circuits for obtaining `a constant direction current from a rectified nlteralternating current, I prefer to -use the system which has been described and claimed in the copending application of Arthur W. Vance entitled Regulating devices, Serial No. 696,001, which was led October 31, 1932, and assigned to the same assignee asthe present application. It should `be understood that any suitable current source may be used for the anode circuits of the several tubes employed in the beat frequency oscillator.

A secondary 81 is used circuits of several tubes.

to energize the heater A connection from this secondary is made to the heater circuit ofthebeat frequency indicator tube 89. This tube is preferably of the type commercially known as the 6E5 which is sold under the trade-mark name Magic Eye. The grid circuit of the indicator 99 is connected through a resistor 9i to a potentiometer. 93.4 The potentiometer 93 is connected through a capacitor 95 to the audio frequency output transformer 1|. The cathode of the indicator 89 is connected to a suitable point on potentiometer 91 which is shunted across the heater-circuit of theindicator. The purpose of this'connection is to obtain a voltage from the 60-cycles per second supply current which is substantially equal and opposite to the current supplied from the output transformer l I. The anode 6| are connected respectively currents generated by the the secondary 13 to maintain a constant voltage 3 current supply may be obtained by connection to the potentiometer 85. The fluorescent screen is represented by reference numeral 30. The operation of the beat frequency indicator vhas been previously described and the patterns obtained are shown in Figs. 2a to 2e, inclusive.

Thus,vI have described a frequency indicator used in conjunction with a beat frequency oscillator in which the currents from twov oscillators are impressed upon a push-pull demodulator. The output of the demoduiator includes radio frequency and audio frequency components. The audio frequency is separated from the radio frequency currents by a lter. The output of the filter, which includes the desired audio frequency current only, is impressed upon an audio frequency amplifier. The several thermionic tubes used in a beat frequency oscillator have heaters which are energized by alternating currents of known frequency. This current of known frequency, together with the audio output current of the beat frequency oscillators, is impressed on the frequency indicator to thereby enable the user to adjust the oscillators to a previously calibrated scale. The several patterns produced by harmonic frequency currents have been described. It should be understood that the beat frequency indicator is not limted to any particular form of area trace. For example, the trace may be the area bounded by clrcles'of fixed or varying radii or a portion of such area.

I claim as my invention:

1. A frequency comparator for indicating the frequency of an unknown alternating voltage with respect to a known alternating voltage, the frequencies of which are related in ratios of small whole numbers, comprising an indicator having a .fluorescent screen, a control electrode means for producing indications of distinguishably different luminescence -on said screen, the' potential of said control electrode means determining the dis- 7tinguishable indications, and means for varying the instantaneous potential of said control electrode means as a function of the sum of said known and unknown voltages. l

2. A frequency comparator for indicating the frequency of an unknown with respect to a known alternatingv voltage, the frequencies of which are related in ratios of small whole numbers, comprising an indicator having a fluorescent screen, a control electrode' radially disposed with respect to said screen for producing sectors of distinguishably different luminescence on said screen, the potential of said control electrode determining the relative areas of said sectors, and means for varying the instantaneous potential of said control electrode as a function of the sum of said known and unknown voltages.

3. A frequency comparator for indicating the frequency of Aan unknown alternating voltage with respect to a known alternating voltage, the frequencies of which are related in ratios of small Whole numbers, comprising an indicator having a fluorescent screen, a control electrode radially disposed with respect to said screen, and means for varying the potential of said control electrode as a function of the sum of said known and unknown voltages to produce sectors of distinguishably different luminescence on said screen, t e number and relative luminescence of said sectors being a measure of said frequency ratio.

4. A frequency comparator for indicating the relative frequencies of two voltages the frequencies of which are related to each other in ratios of small whole numbers, comprising an indicator )having a control electrode and a uoresoentscreen, means for producing sectors having dif- .ferent degrees of luminescence on said screen, means responsive to the instantaneous potential of said control electrodefor varying the relative areas of luminescence oi' said sectors, and means for varying the instantaneous potential of said electrode as a. function of the vector sum of said -two voltages whereby said 'screen is divided into luminescent sectors, the number of said sectors being dependent on said frequency ratio.

' RANALD D. SCHELDQRF. 

